April 30, 2020

Not All Olympic Convergence Zones Are Located over Puget Sound

In this blog I talk a lot about the Puget Sound convergence zone, in which air moves around the Olympic Mountains and converges over Puget Sound, producing a band of clouds and precipitation somewhere between Everett and Seattle (see figure from my NW weather book)


But sometimes, when the wind approaches the coast from a more southwesterly direction, the convergence line can shift northward, producing a line of precipitation from roughly Port Townsend to Bellingham.  This is exactly the situation that occurred this morning.

The radar image at 6:46 AM this morning shows the story, with moderate (green) to heavy (yellow) rain in a band stretching to the northeast.  Was really raining hard in parts of Bellingham and in the hills to the northeast of town!


The visible satellite imagery about an hour later is pretty dramatic--you can see the cloud band stretching from the Olympics northeastward and profound rainshadowing north and south, with essentially clear skies in the lee of the mountains of Vancouver Island and from north Seattle southward.


I am proud to report that the UW WRF model predicted this precipitation band in the run started 5 PM on Wednesday...check it out below (three hour precipitation total ending 5 AM today)


The precipitation totals from midnight to noon today shows the results of the convergence band with some of the hills northeast of Bellingham getting over and inch.


In fact, Heather Meadows at Mt. Baker enjoyed a torrent of 1.94 inches!


To get such a precipitation band, the low-level, larger-scale flow approaching the Olympics from off the ocean needed to be from the southwest, something confirmed by the winds around 5000 ft (850 hPa pressure) forecast by the WRF model at 5 AM.



Finally, I should note that with recent rains, the April total will not end up anywhere near record dry territory.  The first half of the month was bone dry, but recent rains have brought us comfortablly close (but still below) to normal (see Seattle's cumulative April rainfall below, cyan is normal, purple is this year):
More rain is coming, particularly on Saturday.  Sunday looks decent.






5 comments:

  1. And on many occasions I'm sure I've seen an active convergence zone downwind of Vancouver Island. This can happen when the winds are parallel to the "long" dimension of the Island, ie, from the northwest.

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  2. is there a southern convergence zone from the Chehalis gap

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  3. I've always thought Bellingham enjoys less foggy days (than those on Puget Sound) due to the wind coming off the strait. Is there anything to that?

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  4. My favorite convergence zone incident as told by Cliff is about one that formed over the ocean SW of the Olympic range. Cliff showed that it had been forecast. Winds from the NE, of course.

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